1. Field of the Invention
The present invention relates generally to devices for detecting an edge of material in a textile folding or guiding device and, more particularly, a device for detecting the edge of fabric in a folder for such sewing machines, such as a felling folder.
2. Description of the Prior Art
There are many critical applications in the manufacture of apparel. One particular application is the manufacture of lap seams, the type normally used in jeans, work clothes, and similar articles of apparel. The usual method of making such lap seams has been to feed the seam edges of two pieces of fabric through a 2-section folder to progressively fold such edges of material in an interlocking fashion. Two rows of stitches, each adjacent to one of the edges of the formed seam, are then added. If an excess width of material is fed into the folder, one or both pieces of material may be double-folded resulting in a roping effect or excessively thick lap seams. On the other hand, if an insufficient width of fabric is inserted in the folder, the folded edges of the material fail to interlock and a thin seam results. This condition, in which the short, underfolded material tends to snap out or unfold between the time the fabric leaves the folder and the time the seam is stitched, is called underloading. It is desirable to prevent the raw edges of fabric which occur with underloading since this condition results in a second quality garment.
In the past, it has been necessary to rely on the skill of the operator in order to produce the lap seam correctly. As a result, the felling operators are the highest paid workers in the sewing operation. Unfortunately, since such seams are often used in tube construction, the operator has no way of visually determining if the seam is made correctly and must rely on the "feel" of the garment as it is being made.
Some prior art folders have attempted to use mechanical means in controlling the loading condition. One example of such a mechanical device is disclosed in U.S. Pat. No. 4,395,963 issued to Diacont et al. However, such mechanical devices are known to be particularly delicate and are susceptible to malfunction. Also mechanical devices are subject to wear as well as vibration. Finally, these types of devices are too insensitive to detect the movement of a single-ply workpiece and do not provide the reliability required in these operations.
It also has been known to use optical scanners in applications which require the generation of a signal when a workpiece arrives at a predetermined location, such as for scanning flat, sheet-like material workpieces as they progress through a sewing machine. Such devices detect the edge of a piece of material on a reflective work surface having a coefficient of reflection different from the material workpiece being processed when the material interrupts a beam of light. Such devices have an emitter and a detector in positions for the detector to receive the reflection by the work surface of light emitted by the emitter, generally on the sewing head above the material being sewn on the sewing machine. Thus, this design actually functions as a "folded" transmission mode photocell arrangement. One example of such a device is disclosed in U.S. Pat. No. 4,423,691, issued to Schwaab.
Certain disadvantages become apparent with such a design. Reflection from a shiny surface is mostly specular, rather than diffused (i.e., the angle of incidents is equal to the angle of reflection). As a result, the sensor must be mounted almost exactly perpendicular to the shiny surface for reliable sensing. Secondly, the requirement for precise positioning also makes such devices particularly sensitive to vibration. Finally, because these devices detect the edge of a piece of material when the beam of light reflected back from the work surface is interrupted, such devices are unable to detect the presence of fabric work pieces which have a coefficient of reflectivity similar to the surface of the work surface. As a result, such devices are particularly unreliable when fabric is highly reflective, such as white cloth, or when the front and back of the material have different reflectivities, such as denim.
It has thus become desirable to develop an edge detector for detecting the passage of the edge of a workpiece through a sewing machine which does not require precise positioning of the detector. In addition, it has become desirable to develop a edge detector which is insensitive to vibration. Finally, it has become desirable to develop an edge detector which is able to detect the edge of fabric work pieces having a wide degree of reflectivity and which is simple to install, use, and maintain.